1. Field of the Invention
The present invention relates to a clutch plate, more particularly, to a clutch plate used for a drive force transmission device installed in a four-wheel drive vehicle.
2. Description of the Related Art
As an example of a drive force transmission device, it is disclosed in Japanese Patent Publication No. 7-71563. In this publication, it is disclosed that a clutch is arranged between outer and inner rotating members which are coaxially positioned relative-rotatably with each other. In the drive force transmission device, a torque is transferred between the outer and inner rotating members by frictionally engaging the clutch.
In the drive force transmission device disclosed in the publication, faced surfaces with each other of both inner and outer clutch plates constructing the clutch form to a frictional engage surface having a predetermined surface roughness, so that the torque can be transferred between the inner and outer rotating members by the frictional engage force on the frictional engage surface. Further, lubricating oil for lubricating the frictional engage surfaces slidably contacted is supplied thereto.
In the drive force transmission device described in the above-mentioned publication, plural eccentric grooves are formed on the frictional engage surface of one of the inner and outer clutch plates constructive of the clutch, while that of the other thereof is formed to be flat. In such a situation, the decrease of frictional coefficient .mu. by the wedge operation in oil can be prevented so as to keep the lubricating oil at a minimum filling in each of the eccentric grooves.
Further, in the drive force transmission device described in the above-mentioned publication, an interval distance between the adjacent two eccentric grooves formed on the frictional engage surface of one of the clutch plates is relatively widely formed. In a case that the eccentric groove is relatively new, it is possible to keep a predetermined surface roughness on the frictional engage surface, so that the lubricating oil is kept in the depression portion due to the surface roughness. Consequently, the clutch plates can be frictionally engaged with each other at the convex portion adjacent to the eccentric grooves. However, the convex portions adjacent to the eccentric grooves are specularly ground due to the long-term use of the drive force transmission device, so that the oil film is formed between the both frictional engage surface with facility. As a result, it is difficult to engage the clutch plates with each other.
Further, in a case that the concave-convex shape is still remained on the frictional engage surface due to the surface roughness, the lubricating oil is kept in space portions formed by the concave-convex shape on the frictional engage surface. In this situation, the oil pressure separating the frictional engage surfaces each other is generated due to the wedge operation of the lubricating oil kept therein at a portion where the space portions cross in a rotational direction, so that the frictional coefficient .mu.is decreased. Therefore, the function in the multi-disk clutch is deteriorated.
With this deterioration of function in the clutch, in a case that the rotational speed difference between the both clutch plates is small, the torque transfer characteristic becomes to be a negative inclination, i.e., the transmissive torque is gradually decreased as the rotational speed difference becomes to be large.
Further, between the both clutch plates, a gap exists due to the dispersion of the concave-convex shape on the frictional engage surface. In such a frictional clutch of electromagnetic type wherein the frictional clutch is frictionally engaged by the operation of magnetic force, the gap causes to decrease the operative magnetic force thereon, so that the frictional engage force of the frictional clutch decreases. Moreover, the gap does not keep at constant due to the dispersion of flatness (surface roughness) on each of the frictional clutches. Consequently, the dispersion of gap causes that of the torque transfer characteristic product by product in the frictional clutch of the electromagnetic type.